Many contemporary engine controls have integral misfire detection systems. A misfire in an engine will lower efficiency and raise emissions due to poor combustion. With ever-increasingly more stringent legislated emissions standards, the assurance of accurate and complete misfire detection under all engine and vehicular operating conditions is becoming mandatory. In particular, the industry has set next generation standards for On-board Diagnosis (OBDII) for detection of engine misfire. These new standards will require that many different types of misfires must be detected up to engine redline.
Commonly, system designers rely on measurement of engine acceleration, dependent largely on engine torque produced (or not produced) during a combustion process to determine misfiring of a particular engine cylinder. Given the acceleration information, misfires are predicted by various signature analysis, and/or spectral analysis, methods.
As a practical matter, an engine's acceleration behavior is also affected by powertrain related behaviors other than firing torque. These other behaviors can significantly reduce fidelity or signal-to-noise ratio (SNR) of the primarily firing torque related acceleration signal under analysis. Furthermore, under some engine operating conditions, the noise exceeds the primarily engine torque related acceleration signal under analysis. Moreover, the noise related behavior is not limited to only engine operation causes, but include behaviors related to the complete driveline. Some noise related behaviors that are detrimental include relatively low frequency, or firing rate, driveline resonance effects, or vibrations, excited at least partially by cylinder misfiring, torque converter lockup, low speed lugging behavior characteristic of a manual transmission, a change in transmission gears, rough road conditions, etc. Each of these sources of stimulus excite the driveline to perturbate, or transiently oscillate, at a resonant frequency or harmonic thereof. Other noise related behaviors that are detrimental are of a higher frequency, such as occurs when an engine operates at a high engine speed, under fueled conditions, and under various loading, which can all mask engine misfires. These sources of noise may be random in nature.
When the above-mentioned behaviors manifest themselves a significant measure of what amounts to noise, relative to the misfire induced behavior, is introduced into the acceleration measurement. This noise can largely swamp out any signatory behavior of a misfire event.
FIG. 1 shows a first portion 101 of a noise-free waveform indicative of an acceleration signal derived from an engine's crankshaft due to a properly firing cylinder, firing in a sequence of several cylinders, and a second portion 103 of the waveform indicative of acceleration of an engine's crankshaft due to a misfiring cylinder later in the sequence of firing cylinders. At reference number 103 the engine's crankshaft grossly decelerates because proper firing did not occur. Given this observation of acceleration behavior, a magnitude comparison process can monitor the engine's acceleration behavior at a predetermined threshold 105 and indicate a misfiring condition if the acceleration signal transitions below the threshold 105.
FIG. 2 illustrates a behavior of an actual acceleration signal 201 derived from a running engine over about one-hundred fifty cylinder combustion cycles. This acceleration signal 201 includes a repetitively induced misfire by periodically removing a spark signal from one cylinder. From FIG. 2 it can be seen that in a real-world application, the signal derived from a running engine is affected by causes other than combustion related torque as asserted earlier. For reference purposes, the reference markers associated with the horizontal axis 203 demarcate the repetitively induced occurrences of misfire. The waveform 201 is derived using an acceleration sensing device coupled to the engine's crankshaft. Because of crankshaft torsional vibrations, inertial torque due to reciprocating masses, driveline resonance effects, and other mechanically induced vibrations on the engine's crankshaft, the waveform shown in FIG. 2 has relatively poor fidelity. Not only are there the harmonic effects due to the periodic misfire there are also random events which essentially produce noise. The situation further deteriorates at higher engine speeds and during different engine fueling and/or loading conditions. These different conditions make detection of misfire by a simple threshold detection scheme substantially hopeless.
Some misfire determination schemes use running average filters and/or mean or median filters to eliminate low frequency behavior—such as driveline vibration behavior in an acceleration signal. Running average filters are somewhat adequate for smoothing random non-impulsive perturbations in the incoming signal but tend to smear sharp monotonic edge transitions that occur due to driveline inputs, whereas median filters tend to preserve the sharp driveline edge transitions while rejecting impulsive inputs (e.g., misfire acceleration behavior) but are more influenced by non-impulsive variations. Moreover, these techniques do not address severe torsional oscillations or noise due to high speed conditions, fueled conditions, different loading conditions, multiple misfires per engine combustion cycle, etc.
What is needed is an improved approach for misfire detection that can detect a broad range of misfire patterns, particularly one that is insensitive to adverse powertrain operating effects. In particular, an improved system needs to account for driveline perturbations over a wide range of engine operating conditions including operation up to an engine speed redline. This improved technique also needs to improve acceleration signal fidelity by improving the acceleration signal's signal-to-noise ratio in order to accurately detect misfire. The improved technique ideally would detect both periodic, random misfires, and multiple misfires per engine combustion cycle in an acceleration signal with high fidelity.